Enhancement of Phytosterol and Triterpenoid Production in Plant Hairy Root Cultures—Simultaneous Stimulation or Competition?

Abstract

Plant in vitro cultures, including hairy roots, can be applied for controlled production of valuable natural products, such as triterpenoids and sterols. These compounds originate from the common precursor squalene. Sterols and triterpenoids distinctly differ in their functions, and the 2,3-oxidosqualene cyclization step is often regarded as a branch point between primary and secondary (more aptly: general and specialized) metabolism. Considering the crucial role of phytosterols as membrane constituents, it has been postulated that unconstrained biosynthesis of triterpenoids can occur when sterol formation is already satisfied, and these compounds are no longer needed for cell growth and division. This hypothesis seems to follow directly the growth-defense trade-off plant dilemma. In this review, we present some examples illustrating the specific interplay between the two divergent pathways for sterol and triterpenoid biosynthesis appearing in root cultures. These studies were significant for revealing the steps of the biosynthetic pathway, understanding the role of particular enzymes, and discovering the possibility of gene regulation. Currently, hairy roots of many plant species can be considered not only as an efficient tool for production of phytochemicals, but also as suitable experimental models for investigations on regulatory mechanisms of plant metabolism.

Modulation of Steroid and Triterpenoid Metabolism in Calendula officinalis Plants and Hairy Root Cultures Exposed to Cadmium Stress

“…As it has been frequently reported for toxic metals and metalloids, high Cd concentrations in plants can alter metabolic processes and exert various effects at the physiological, morphological, and molecular levels, e.g., growth inhibition, nutrition imbalance, photosynthesis suppression, chlorosis, and ROS (reactive oxygen species) increase [ 21 , 22 ]. Various reports indicate that the composition of steroids and triterpenoids might be modified under stress conditions; however, detailed studies on such effects exerted after exposure to heavy metals are scarce [ 11 , 23 , 24 ]. The first reported studies on this phenomenon, performed either on algae or plants, demonstrated effects such as increased levels of cholesterol and a decrease in the ratio of 24-ethylcholest-5-en-3β-ol to 24-ethylcholesta-5,24(28) Z -dien-3β-ol, two major sterols occurring in the marine diatom Asterionella gracilis [ 25 ], or the increase of stigmasterol and sitosterol contents parallel to the decrease of isofucosterol and campesterol, as observed for rice ( Oryza sativa cv.…”

“…Indeed, in a previous study, the reported increase in the dry weight of mercury-treated algal cells appeared to be uncoupled from cell division [ 25 ]. Moreover, the inhibition of sterol biosynthesis, according to the “plant Cornelian dilemma” of the growth-defense trade-off [ 24 ], can significantly induce the biosynthesis of triterpenoids or other compounds excessively accumulating in cells under stress conditions…”

“…A common phenomenon observed in plants subjected to stress conditions is the competitiveness of the primary/general and secondary/specialized metabolic pathways, including those of steroids and triterpenoids with a common precursor, namely, squalene [ 24 ]. Some symptoms of this competition could be observed in the present study, i.e., the increase in sterol biosynthesis parallel to the decrease in the triterpenoid content in C. officinalis plant roots and hairy root culture, as well as the inverse phenomenon, namely, the decrease in the sterol content accompanied by the stimulation of triterpenoid biosynthesis in shoots.…”

Effect of Ethylene and Abscisic Acid on Steroid and Triterpenoid Synthesis in Calendula officinalis Hairy Roots and Saponin Release to the Culture Medium

“…Sterols and triterpenoids distinctly differ in their functions, and therefore, they are commonly regarded as general and specialized metabolites, respectively [ 21 ]. Since both sterols and triterpenoids are synthesized as products of one common precursor, 2,3-oxidosqualene, the trials of stimulation of triterpenoid biosynthesis in plants and plant in vitro cultures including hairy roots often concern the problem of the possible competition [ 22 ].…”

The Influence of Exogenous Jasmonic Acid on the Biosynthesis of Steroids and Triterpenoids in Calendula officinalis Plants and Hairy Root Culture

“…Plants’ acclimation to environmental changes, as well as defense strategies against biotic and abiotic stress factors, involve a complex metabolic remodeling triggered by signaling molecules such as jasmonic acid [ 1 , 2 ]. It is generally postulated that the reallocation of energy and metabolic resources drives the antagonism between plant growth and defense, resulting in pathway competition between general and specialized metabolism [ 3 , 4 ]. In the context of the growth-defense trade-off, the course of metabolic modifications of steroids and triterpenoids is particularly intriguing.…”